the *default* BUILD is a method so other classes can inherit it if they don't with to define their own, but that is because the default BUILD is written generically. the assumption is that any BUILD defined by a subclass will have specific knowledge about that class, and therefore should be a submethod

on the subject of registering things in the grammar, you can't modify the grammar by registration, only by derivation, or you essentially have monkey-typed languages in a lexical scope, which p6 prohibits